Boeing Co workers at a C-17 plant in Long Beach, California, will return to the job on Monday, ending a one-month strike, but Boeing faces another possible work stoppage at a plant in St. Louis.

About 1,700 workers represented by the United Aerospace Workers Union went on strike on May 11 in a contract dispute. Union members ratified a new deal on Wednesday, according to the UAW Local 148 website.

Meanwhile, leaders of the International Association of Machinists and Aerospace Workers, which represent workers at Boeing's St. Louis Defense plant, has advised members to reject the company's most recent contract proposal and prepare for a strike.

Several issues that went unresolved forced the bargaining committee to take a strong stand against the proposed contract, IAM said on its website.

The union, which represents about 2,500 employees in St. Louis, balked at language in the proposal that it says would exclude future members from the defined pension plan in which current members are included. An IAM spokesman said that if the workers reject the deal, the union will ask for a vote granting union leaders the authority to call a strike.

The soonest that work could stop is June 21, IAM spokesman Thomas Pinski said.

Employees at Boeing Defense, Space, & Security in St. Louis work on F/A-18E/F Super Hornets for the U.S. Navy and the Royal Australian Air Force, EA-18G Growlers for the U.S. Navy, F-15 Strike Eagles for South Korea and Singapore.

The facility also makes parts for the C-17 aircraft. Production lines on the current programs extend through the next five years.

Boeing said in a statement that its contract proposal is fair and equitable to the employees in St. Louis. A company spokesman declined to say how disruptive a strike would be to operations there.

We're making contingency plans if that is the case, he said.

Boeing shares were up 0.8 percent at $64.40 on the New York Stock Exchange on Friday afternoon.

(Reporting by Kyle Peterson, editing by Matthew Lewis)